FOCUS J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 1Present Address: Division of Entomology and Nematology, Indian Institute of Horticultural Research, Bangalore 560089, India Management of potato nematodes: An overview K. S. Krishna Prasad1 Central Potato Research Station Ootacamund-643004, Nilgiris District, India E-mail: karpakal@yahoo.com ABSTRACT Root-knot nematodes and cyst nematodes are important constraints that reduce potato yields in India. Three species of Meloidogyne cause root-knots on the crop throughout the country, of which, M. incognita is more wide-spread. Infected tubers also result in marketable-yield-loss particularly in the seed potatoes. The cyst nematodes include two species of Globodera restricted to the hilly regions of Tamil Nadu and are of quarantine importance, inhibiting seed- potato production. Potato produce from these hills is used only for consumption. The endoparasitic nature of their life cycle, deposition of eggs into a gelatinous egg mass in root knot and the female turning in to a hard cyst encompassing the eggs within them in cyst nematode makes them difficult organisms to manage. Both these nematodes exhibit physiologic variation, hence, their management is not absolute with host-resistance. Therefore, an Integrated Nematode Management (INM) is adopted in both the cases. Root-knot nematode in North India is managed using nematode-free seed tubers, crop rotation with maize or wheat and application of 1-2 kg ai /ha Carbofuran 3% G at the time of potato planting. Cyst nematode in Tamil Nadu hills is managed by crop rotation with vegetables, particularly cabbage and carrot, intercropping potato with beans or wheat, alternating nematode resistant potato variety ‘Kufri Swarna’ and application of 2 kg ai /ha Carbofuran 3% G at planting. A two-year adoption of INM for root-knot and a three-year INM practice for cyst nematodes gives efficient and economical production system. Potato farmers in Himachal Pradesh and Tamil Nadu hills follow practices standardized at the Central Potato Research Institute, Shimla and it’s sub- station in the Nilgiri hills. Key words: Root-knot nematodes, cyst nematodes, late blight disease, pathotypes, virulent strains, host resistance, crop rotation, cropping sequence, integrated management INTRODUCTION Cultivated potato Solanum tuberosum Linn., originated in the Andes mountains in South America, was introduced in the 16 th century to Europe and was subsequently distributed throughout the world (Pushkarnath, 1976). Now, it is grown in almost all countries and is recognized as the world’s most important tuber crop playing a vital role, meeting food requirement of people, particularly, in the developing countries (Swaminathan and Sawyer, 1983). Being one of the top high-value crops, its importance is highlighted by the United Nations by naming the year 2008 as the ‘International Year of Potato’ (FAO, 2007). Its global production is about 320 million tones, of which China (72 million tonnes) Russian Federation (35 million tonnes) and India (26 million tonnes) are the major producers. In our country, potato is cultivated in almost all States under varying ecological situations. Basically, potato is cultivated in colder regions, but the adaptability of this crop to varied climates has been well-exploited indigenously by developing varieties suitable to different agro-climatic zones in India (Pushkarnath, 1976). Thus, the crop is grown under long-day conditions of summer months in the mid- hills of Himalayas, short-day conditions of winter months in the North-West plains of Punjab, Haryana, Uttar Pradesh, Bihar and West Bengal, in the equinox conditions of Deccan Plateau during rainy seasons, and, almost round-the-year in equable climate of Tamil Nadu hills (Grewal et al, 1992). It now serves as a major food source next only to rice, wheat and maize in our country (Pandey and Sarkar, 2005). Major constraints in potato production are insect pests, nematodes and diseases which account for nearly 37% yield loss throughout the globe, of which the share of 90 J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 diseases and nematode parasites alone is 23% (Sasser and Freckman, 1987). Late blight (Phytophthora infestans), bacterial wilt (Ralstonia solanacearum), tuber moth (Phthoremia operculella), root-knot nematodes (Meloidogyne spp.) and cyst nematodes (Globodera spp.) are major pests, especially in India. But, unlike the infestation or incidence of an insect pest or disease, nematode infections are difficult to recognize or diagnose, as, these are often mistaken for nutrient deficiency. Primarily, the root-knot nematode (RKN) and potato cyst nematode (PCN) are important among 90 parasitic nematodes (Table 1) associated with the potato rhizhospere in India (Krishna Prasad, 1993). Both these nematodes are highly-adapted, obligate plant parasites exhibiting endoparasitic life cycle. Other important nematode parasites (Fig.1) that occur in the potato rhizoshere are spiral (Helicotylenchus spp), stunt (Tylenchorhynchus spp; Quinsulcius spp.), lesion (Pratylenchus spp.), reniform (Rotylenchulus reniformis) and pin (Paratylenchus spp.) nematodes, that feed on potato roots, causing appreciable yield-reduction (Krishna Prasad and Sharma, 1985; Krishna Prasad and Rajendran, 1990). Economic importance The earliest record of a plant parasitic nematode (PPN) infection on potato was that of PCN by Julius Kuhn in 1881 at Rostoch, Germany, followed by occurrence of RKN by Neal in 1889 at Florida, USA. Since then, at least 150 species of PPN have been encountered in the rhizosphere of potato throughout the world (Jensen et al, 1979). Among these, the RKN (Meloidogyne spp. - basically tropical, polyphagous and prevalent in all regions) and PCN (temperate, host-specific and mainly restricted to potato-growing localities in mild climates) are the most important nematode parasites of potato (Jones et al, 1981). The endoparasitic nature of life cycle and protection of eggs in an egg-mass, or a cyst, make these organisms difficult to manage (Sethi and Gaur, 1986). Between these two, the PCN, popularly called ‘The Golden Nematode’, is important throughout the world and is considered as the number one quarantine pest (Trudgill, 1985). Most of the countries free of PCN have quarantine regulations on import of potato that might endanger introduction of cysts into their country (Stone, 1985). Domestic quarantine regulations are strictly followed in the USA, Canada and India to prevent further spread of PCN (Brodie et al,1993). Crop loss Nematodes are mostly root feeders living in soil and cause gradual yield reduction, in addition to predisposing plants to infection by other microorganisms. The degree of damage depends on crop husbandry, nematode density and environmental conditions (Evans, 1993).The estimated yield-loss in potato by PPN around the world is 12.2% (Sasser, 1989). Under Indian conditions, an initial level of even two larvae of RKN per gram of soil results in overall yield reduction of 42.5% while, PCN accounts for 65% loss (Krishna Prasad, 1993). RKN infection on tubers manifests, as pimple-like blisters. Mere presence of two infected tubers per bag of 80 kg seed potato is sufficient to reject for export from Himachal Pradesh, a state which follows potato seed certification (Krishna Prasad, 1986). Quarantine regulations Occurrence of PCN in India on potato from Nilgiri hills during 1961 provided the trigger for organized nematological research in the country, as, this nematode had established itself as one of the most destructive pests of potato all over the world (Evans and Stone, 1977; Seshadri, 1978). Potential danger from this nematode to potato cultivation in the country was such that Government of Tamil Nadu amended the Destructive Insect Pest Act 1914 in 1971 to ensure inspection of seed potato for presence Fig 1. Major parasitic nematodes associated with potato in India Krishna Prasad 91 Table 1. Parasitic nematodes associated with the potato crop in India Nematode Locality State Initial record Anguina tritici Kufri Himachal Pradesh CPRI, 1975 Aphelenchoides avenae Kufri Himachal Pradesh CPRI, 1985 A. ritzemabosi Kufri Himachal Pradesh CPRI, 1975 A. solani Kufri Himachal Pradesh CPRI, 1965 Aphelenchus avenae Hyderabad Andhra Pradesh Das, 1960 Aligarh Uttar Pradesh Khan et al, 1964 Jalandhar Punjab CPRI, 1974 Shimla Himachal Pradesh CPRI, 1985 Criconemoides ornatus Kufri Himachal Pradesh CPRI, 1960 Criconemoides spp. Kufri Himachal Pradesh CPRI,1957 Ditylenchus destructor Shillong Meghalaya CPRI, 1965 Imported tubers NBPGR, 1980 D. dipsaci Imported tubers NBPGR, 1980 D. solani Hapur Uttar Pradesh Hussain and Khan, 1976 Ditylenchus spp. Aligarh Uttar Pradesh Fasahat et al, 1973 Dorylaimus spp. Shimla Himachal Pradesh CPRI, 1965 Ecphydophora goodeyi Aligarh Uttar Pradesh Hussain and Khan, 1965a Enchoderella mustafi Aligarh Uttar Pradesh Hussain and Khan, 1965 Eudorylaimus monohystera Aligarh Uttar Pradesh Jairajpuri, 1969 Globodera pallida Ootacamund Tamil Nadu Howard, 1977 Devikulam Kerala Ramana and Das, 1988 G. rostochiensis Ootacamund Tamil Nadu Jones, 1961 Kodaikanal Tamil Nadu Vijayarhagavan et al, 1975 Helicotylenchus caudatus Almora Uttaranchal Sultan, 1985 H. crenatus Bhakulty Himachal Pradesh CPRI, 1966 H. dihystera Bhakulty, Himachal Pradesh CPRI, 1965 Keylong Himachal Pradesh CPRI, 1985 H. nannus Kufri Himachal Pradesh CPRI, 1960 H. willmottae Ootacamund Tamil Nadu Siddiqui, 1972 Helicotylenchus spp. Ootacamund Tamil Nadu Murthy, 1963 Shillong Meghalaya CPRI, 1965 Delhi Delhi Khan and Wadhwa, 1969 Jalandhar Punjab CPRI, 1974 Kufri Himachal Pradesh CPRI, 1976 Imported tubers NBPGR, 1980 Hemicriconemoides spp. Aligargh Uttar Pradesh Fasahat et al,1973 Shimla Himachal Pradesh CPRI, 1985 Jalandhar Punjab CPRI, 1985 Hemicycliophora spp Kufri Himachal Pradesh CPRI, 1960 Imported tubers NBPGR, 1980 Heterodera avenae Shimla, MandiKulu, Himachal Pradesh Krishna Prasad, 1986 SirmourKinnaur and Lahaul-Spiti H. carotae Himachal Pradesh Swarup et al, 1964 H. punciata Himachal Pradesh CPRI, 1966 Hoplolaimus galetus Patna Bihar CPRI, 1960 H. indicus Aligarh Uttar Pradesh Fasahat et al, 1973 Hoplolaimus spp. Allahabad Uttar Pradesh Edward et al, 1963 Delhi Delhi Khan and Wadhwa, 1969 Aligarh Uttar Pradesh Fasahat et al, 1973 Jalandhar Punjab CPRI, 1974 Imported tubers NBPGR, 1980 Management of potato nematodes J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 92 Indokochinema ekramullahi Burdwan West Bengal Jana and Baqri, 1982 Lelenchus annulatus Octacamund Tamil Nadu Saddiqui and Khan, 1983 Longidorella minitissima Aligarh Uttar Pradesh Khan, 1972 Longidorus elongetus Aligarh Uttar Pradesh Khan et al, 1964 L. nirulai Shillong Meghalaya Siddiqui, 1965 Longidorus spp. Shillong Meghalaya CPRI, 1965 Kufri Himachal Pradesh CPRI, 1966 Aligarh Uttar Pradesh Fasahat et al, 1973 Jalandhar Punjab CPRI, 1975 Meloidogyne arenaria Aligarh Uttar Pradesh Khan et al, 1964 M. hapla* Shimla Himachal Pradesh CPRI, 1975 M. incognita* Kufri Himachal Pradesh Thirumalchar, 1951 M.javanica* Patna Bihar Pushkarnath and Roy Choudhary, 1958 Meloidogyne spp. Imported tubers NBPGR, 1980 Michonchus digiturus Rajgurunagar Maharashtra Jairajpuri, 1969 Nothotylenchus cylindricus Almora Uttaranchal Khan and Siddiqi, 1968 N. geraerti Aligarh Uttar Pradesh Khan and Siddiqi, 1968 N. hexaglyphus Almora Uttaranchal Hussain and Khan, 1974 Ogma spp. Srinagar Jammu & Kashmir Paratrichodorus spp. Almora Uttaranchal Khan, 1972 Paratylenchus spp. Shimla Himachal Pradesh CPRI, 1973a Jalandhar Punjab CPRI, 1974 Imported tubers NBPGR, 1980 Ootacamund Tamil Nadu Krishna Prasad, 1986 Pratylencuhus brachyurus Kufri Himachal Pradesh CPRI, 1962 P. brevicauda Hyderabad Andhra Pradesh Das, 1960 Shillong Meghalaya CPRI, 1965 Kufri Himachal Pradesh CPRI, 1966 P. coffeae Allahabad Uttar Pradesh Edward, 1969 Shimla Himachal Pradesh CPRI, 1985 P. indicus Hyderabad Andhra Pradesh Das, 1960 Shillong Meghalaya CPRI, 1965 Kufri Himachal Pradesh CPRI, 1966 P. penetrans Shimla Himachal Pradesh CPRI, 1962 P. pratenses Shillong Meghalaya CPRI, 1965 Kufri Himachal Pradesh CPRI, 1966 P. teres Jallandhar Punjab Khan and Singh, 1975 Allahabad Uttar Pradesh Edward et al, 1963 Pratylencuhus spp. Ootacamund Tamil Nadu Murthy, 1963 Aligarh Uttar Pradesh Fasahat et al, 1973 Shimla Himachal Pradesh CPRI, 1974 Madapur Karnataka Singh et al, 1979 Psillenchus spp. Shimla Himachal Pradesh CPRI, 1985 Quinisulcius capitatus Kufri, Himachal Pradesh CPRI, 1974 Bhakhulty Himachal Pradesh CPRI, 1985 Keylong Himachal Pradesh Krishnaprasad and Sukumaran, 1986 Q.acti Shimla Himachal Pradesh Nagesh, 1993 Rotylenchulus reniformis Ootacamund Tamil Nadu Murthy, 1963 Delhi Delhi Verma and Prasad, 1969 Solan Himachal Pradesh Swarup et al, 1967 Aligarh Uttar Pradesh Fasahat et al, 1973 Table 1. Continued Nematode Locality State Initial record Krishna Prasad J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 93 R. stajnabu Aligarh Uttar Pradesh Hussain and Khan, 1965b Rotylenchulus spp. Delhi Delhi Khan and Wadhwa,1969 Imported tubers NBPGR, 1980 Rotylenchus ranpoi Rajgurunagar Maharashtra Darekar and Khan, 1982 Rotylenchus spp. Aligarh Uttar Pradesh Khan et al, 1964 Scutellonema spp Aligarh Uttar Pradesh Khan et al, 1964 Thornedia solani Aligarh Uttar Pradesh Hussain and Khan, 1965c Trichodorus christei Kolar Karnataka CPRI, 1965 T. minor Shimla Himachal Pradesh Siddiqi, 1960 T. nannus Jalandhar Punjab CPRI, 1974 T. pachydermus Jalandhar Punjab CPRI, 1965 T. similes Aligarh Uttar Pradesh Khan et al, 1964 Trichodorus spp. Aligarh Uttar Pradesh Fasahat et al, 1973 Jalandhar Punjab CPRI, 1974 Kufri Himachal Pradesh CPRI, 1984 Tylenchorhynchus brevidens Soil samples Delhi Sethi and Swarup, 1968 Punjab Sethi and Swarup, 1968 Rajasthan Sethi and Swarup, 1968 T. claytoni Shimla Himachal Pradesh CPRI, 1985 Jalandhar Punjab CPRI, 1985 T. cuticaudatus Bhubaneswar Orissa Roy and Das, 1983 T. dubius Shimla Himachal Pradesh CPRI, 1975 T. martini ShimlaPatna Himachal PradeshBihar CPRI, 1960 T. mashoodi Aligarh Uttar Pradesh Khan et al, 1964 T.neoclavicaudatus Soil adhering totubers Uttar Pradesh Khan et al, 1964 Imported potato tubers Mathur et al, 1978 T. swarupi Burdwan West Bengal Singh and Khera, 1978 Tylenchorhynchus spp. Shillong Meghalaya CPRI, 1965 Kufri Himachal Pradesh CPRI, 1966 Delhi Delhi Khan and Wadhwa, 1969 Aligarh Uttar Pradesh Fasahat et al, 1973 Imported tubers NBPGR, 1978 Tylenchus spp. Shillong Meghalaya CPRI, 1965 Kufri Himachal Pradesh CPRI, 1966 Xiphinema americanum Kufri Himachal Pradesh CPRI, 1966 X. index Kufri Himachal Pradesh CPRI, 1962 X. indicum Kufri Himachal Pradesh CPRI, 1966 X. radicicola Kufri Himachal Pradesh CPRI, 1966 Xiphinema spp. Shillong Meghalaya CPRI, 1964 Jalandhar Punjab CPRI, 1974 Madapur Karnataka Singh et al, 1979 Ootacamund Tamil Nadu Krishna Prasad, 1986 *Refer Table 2 for detailed, State-wise prevalence of root-knot nematodes Table 1. Continued Nematode Locality State Initial record of cyst nematode and check its spread to other parts of the country. Large-scale inspection of potato fields around the country revealed that this nematode was restricted to the hills in Tamil Nadu (Logiswaran and Menon, 1965). Therefore, seed movement of potato from these hills is banned and the entire produce is used for consumption purposes only. However, detection of PCN from the neighboring states of Tamil Nadu such as Karnataka (Singh and Krishna Prasad, 1986) and Kerala (Ramana and Mohandas, 1988) calls for strengthening of domestic quarantine. The other nematode pests of potato that merit quarantine importance and are not yet found in the country are the potato rot nematode, Ditylenchus destructor and potato false root-knot nematode, Naccobus aberrans (Renjhen, 1973). Management of potato nematodes J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 94 Distribution RKN, with about 65 species of Meloidogyne, is widely distributed throughout the world causing typical root-galls on crop plants (Brodie et al, 1993). At least ten of these infest potatoes, though, M. incognita, M. javanica and M. hapla are predominant. In India, RKN was first reported on potato in 1951 from Shimla (Thirumalachar, 1951) and is prevalent in all the potato growing regions of the country (Nirula and Roy Choudhary, 1957). Meloidogyne incognita is the dominant RKN species causing galls on roots and tubers, followed by M. javanica. Infestation of M. hapla on potato roots is recorded at higher altitudes of 2000m (above) MSL from hilly tracts of Himachal Pradesh, Jammu & Kashmir, Uttaranchal and Tamil Nadu (Table 2). Detailed surveys undertaken in Himachal Pradesh (Fig 2) and Uttaranchal shows that all these three species affect potato with varying intensity. Meloidogyne incognita is occurring either alone or with the other two species, at Fig 2. Root- Knot Nematode Intensity on Potato in Himachal Pradesh Table 2. Species of Meloidogyne causing root-knots on potato in India State / Union Territory Species Initial record Andhra Pradesh M. javanica Das, 1960 Arunachal Pradesh M. incognita Mishra and Jaya Prakash, 1980 Assam M. incognita CPRI, 1957, 1965 Bihar M. incognita CPRI, 1957, Lal and M. javanica Das, 1957 Delhi M. javanica Prasad et al, 1964 Gujarat M. incognita Desai et al, 1970 Haryana M. incognita CPRI, 1971 M. javanica Himachal Pradesh M. hapla CPRI, 1974 M. incognita Mukhopadhyaya,1970 M. javanica CPRI, 1957 Jammu & Kashmir M. hapla CPRI, 1977 Karnataka M. incognita CPRI, 1957 M. javanica CPRI, 1965 Maharashtra M. incognita CPRI, 1965 M. javanica Manjrekar and Talgeri, 1969 Meghalaya M. incognita Pushkarnath and Roy Choudhary, 1958 Orissa M. incognita CPRI, 1977 Punjab M. incognita CPRI, 1963 M. javanica CPRI, 1974 Rajasthan M. incognita CPRI, 1972 M. javanica CPRI, 1974 Tamil Nadu M. hapla, Gill, 1974 M. incognita Singh et al, 1979 M. javanica Singh et al, 1979 Uttar Pradesh M. incognita CPRI, 1957 M. javanica Gill, 1974 Uttaranchal M. hapla Krishna Prasad, 1993 M. incognita CPRI, 1957 West Bengal M. incognita CPRI, 1971 altitudes ranging 760-2900 m MSL. Infestation of M. javanica was observed at lower altitudes of 410 to 1100 m MSL while M. hapla was restricted to higher altitudes of 1950 to 3300 m MSL and its presence could be ascertained by indicator plant reaction (Krishna Prasad, 1986). Tuber infection manifests as blisters and is invariably associated with M. incognita and M. javanica wherever bacterial wilt is endemic (Nirula and Paharia, 1970; Krishna Prasad and Sukumaran, 1986; Nagesh and Shekhawat, 1997). PCN is restricted to the potato growing localities of about 60 countries, with two species that are characterized by colour of the developing female (Evans and Stone, 1977). Globodera pallida (white or cream- coloured females) is prevalent in 25 countries and G. rostochiensis (yellow cyst nematode) is reported from 57 countries (Brodie et al, 1993). Dr. F. G. W. Jones, Head, Nematology Department, Rothamsted Experimental Station, Harpenden, U.K., on a personal trip to Ootacamund, first detected this nematode in India from a field situated at an elevation of 2125m MSL. Realizing the importance of this problem in potato, the Indian Council of Agricultural Research (ICAR) and the Government of Tamil Nadu launched the ‘Golden Nematode Scheme’ at Ootacamund in 1963 (Seshadri and Sivakumar, 1962). Large-scale inspection of potatoes in the marketing mandies at Mettupalayam (trading centre and rail-head at the foothills Krishna Prasad J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 95 Table 3. Distribution of PCN species in major potato growing localities of Nilgiri hills Locality MSL-Climate No. of potato Occurrence% Occurrence% crops /year G. pallida G. rostochiensis Nanjanadu 2250-Cool Two 0.0 100.0 Mynala 2250-Cool Two 15.0 85.0 Vijayanagaram 2175-Cool Two 28.5 71.5 Kavaratty 2175-Cool Two 0.0 100.0 Thalaiattumandu 2150-Cool Two 25.0 86.0 Fern hill 2150-Ambient cool Two 49.0 51.0 Hullahatty 2100- Ambient cool One 51.0 49.0 Finger post 2100 -Ambient cool Two 26.0 74.0 Adigaratty 2070 -Ambient warm One 94.0 6.0 Kallahatty 1950 -Ambient warm Three 92.5 7.5 Sholur 1950 -Ambient warm Two 81.0 19.5 Thummanatty 1920 -Warm Two 100.0 0.0 Thummanada 1920 -Warm Two 100.0 0.0 Jegathala 1875 -Warm One 100.0 0.0 Milidhen 1850 -Warm One 100.0 0.0 Fig 3. Intensity of Potato Cyst Nematodes in Nilgiri Hills. of Nilgiris) and field-to-field surveys were undertaken (Seshadri, 1970). These studies indicated presence of cysts in potato consignments meant for transportation to Bombay, Calcutta, Cuttack and Poona (Seshadri, 1978). Detailed surveys conducted in other major potato growing areas of Assam, Himachal Pradesh, Karnataka, Punjab, Tamil Nadu and Uttar Pradesh indicated this nematode to be restricted to the Nilgiri hills (Gill, 1974) and Kodaikanal hills (Thangaraju, 1983) of Tamil Nadu. Presence of cysts of PCN in several potato- growing villages was recorded in Karnataka (Singh and Krishna Prasad, 1986) although the species could not be determined, as, the eggs in the cysts were non-viable. Later, G. pallida was observed to be associated with potato at the Devikulam locality in Idukki district of Kerala (Ramana and Mohandas, 1988). Species composition, distribution and intensity (Fig 3) indicated 57% PCN population in the Nilgiris to be that of G. pallida and 43% was that of G. Management of potato nematodes J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 96 rostochiensis. Nematode intensity was high (Table 3) in localities where invariably two potato crops were taken in a calendar year (Krishna Prasad, 2003). Both species differed in their preference for infecting and developing on potato, with the former preferring altitudes of 1550 to 2100 m MSL and the latter preferring localities above 2100m MSL (Krishna Prasad, 2004c). Physiologic specialisation Initially it was thought that PCN populations at the Nilgiris comprised G. rostochiensis pathotype A (Hari Kishore et al,1969). Breeding and screening potato for resistance to cyst nematodes brought to light occurrence of Ro1 and Pa 2 in both species (Howard, 1977). Cultivation of nematode-resistant potato ‘Kufri Swarna’ (derived from Solanum vernei) at different localities in the Nilgiris indicated the presence of other pathotypes (Krishna Prasad, 1996). Differential host-reaction studies have shown that three pathotypes occur in each species at the Nilgiris (Krishna Prasad, 2004). Pathotype Ro1 of G .rostochiensis and Pa 2 of G. pallida are the most prevalent and constitute 75% of total population. Other pathtoype Pa 1 accounted for 15%, followed by Ro2, at 7%. The least prevalent pathotypes Pa3 and Ro5 accounted for only 3% but were able to develop distinctly on some of the differential hosts, indicating their virulence (Krishna Prasad, 2006). PCN populations from Kodaikanal constituted pathotypes Ro1 of G. rostochiensis and Pa 2 of G. pallida. Field symptoms Generally, nematode infestations are not easy to recognize and may be suspected when yellowing of foliage, coupled with stunted growth of the plants in patches (Fig 4) is observed which is often mistaken for nutrient deficiency symptoms. Yield-reduction due to nematode damage is Fig 5. Root knot nematode on potato roots with egg masses Fig 6. Female Cyst Nematodes on potato roots Fig 4. Initial nematode infection progressive and increases year after year. RKN juveniles, on entry into the root, cause typical root galls and affect uptake of nutrients. Similarly, PCN juveniles also enter feeder roots, establish and obstruct uptake of minerals and nutrients, impeding the overall plant growth, but without forming root knots. In both the cases, field symptoms appear after the nematode population in the soil builds up to about 10 eggs and larvae (propagules) per 100 ml of soil. Initially, small patches of poorly growing, pale yellow plants are observed. A closer examination of the root system of such plants show galls of RKN (Fig 5) while, in PCN, one can observe yellow or white, small, mustard-size female nematodes sticking to the roots (Fig 6). Temporary wilting of plants occurs around mid-noon and late evening, while, in heavily infested fields, plants remain stunted, show premature yellowing and poor root development. Reduction in size and number of tubers is also observed. Pimple-like blisters appear on tubers due to RKN, which reduces the marketable value of the produce (Fig 7) especially, the seed tubers. Well-grown cysts on tubers are often seen in fields Krishna Prasad J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 97 Fig 7. Tubers infected by RKN heavily infested with PCN (Fig 8) and cause gradual reduction in yield year after year, with poor quality seed tubers despite good crop-management practices (Ravichandran et al, 2001). Nematode biology Lifecycle and biology of RKN and PCN are similar, with only subtle differences. Since RKN is polyphagous, the larvae readily hatch in soil without presence of any stimulant, while, in the oligophagous PCN, hatching of cysts is initiated by root diffusates. Solanine and alfa-chaconine are glycoalkaloids released by potato roots, which act as PCN hatching stimulants. Absence of potato root exudates is known to force PCN eggs to remain unhatched for 15-20 years. The 2nd stage larvae emerging out of the egg-mass or cyst move actively in soil, to invade roots, and lie parallel to the vascular system. This infection results in formation of giant cells from which nematodes extract nourishment. Female larvae undergo successive molts, increasing each time in size to attain a spheri-cal shape. Adult females remain attached to the roots by their neck, are pear-shaped Fig 8. Potato field heavily infected by nematodes Fig 9. Life cycle of root-knot nematode in potato and measure 0.7 -0.8 mm in diameter and are white or yellow in colour. RKN eggs are laid into a gelatinous matrix (Fig 9) while, in PCN, females turn brown and become hard cysts. Male nematodes are thread-like and come out freely from the root system. Approximately 25 to 30 days are required for completion of life cycle in both the cases (Fig 10). However, in winter months at Shimla, RKN took about 65 days to complete its lifecycle, due to snowfall that decrease the soil temparatures (Nirula and Raj, 1969). Fig 10. Life cycle of cyst nematode in potato Management of potato nematodes J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 98 Generally, one nematode generation is completed in a crop season as potato itself is grown as a sandwitch crop at most places, particularly, in the Indian plains. Evidence of the 2nd generation being completed in both RKN and PCN is available. As there is no distinct dormancy in RKN prior to hatching, larvae from the first generation can infect stolons and tubers, while PCN exhibits specific dormancy. Globodera rostochiensis, with a shorter dormancy of 45 to 60 days can complete its second generation in the Nilgiri hills (Krishna Prasad, 2004b) on potato in the long-duration varieties that complete a crop cycle in 130 days. Globodeva pallida generally has one life cycle, with 60 to 75 days’ dormancy. Multiplication rate of both the nematodes ranges from 7 to 13 times the initial population in one crop cycle on potato (Krishna Prasad, 2004c). Studies have shown that G. pallida is able to develop and reproduce in the foothills of Nilgiris at 300 to 350 m MSL from October to February, when ambient temperatures ranges from 14° to 19°C (minimum) and 22o to 30°C (maximum). However G. rostochiensis could develop into females only at 1400m MSL and above, where maximum day temperature is not more than 24°C during the above months (Krishna Prasad, 2004a). Nematode spread At the time of harvest, egg-masses or the brown cysts containing eggs are easily dislodged into the soil (Fig 11). Nematode infection by either the egg mass or cysts with eggs within usually spreads with soil adhering to the farm implements, harvested tubers, gunny bags, etc. Other major means of spread is through contaminated compost, laborers feet, and, by seed potatoes. During monsoon months, water running down slopes also facilitates transmittance or spread of cysts. ( Logiswaran and Menon, 1965). Fig 11. Mature cysts in soil debris Nematode management Nematode management is practiced using seeds obtained from nematode-free areas and by following several cultural practices like deep ploughing and exposing soil to summer heat, trap cropping, rotation with non-host crops, intercropping in potato based cropping system, nematicide treatment to reduce the initial nematode population, breeding nematode-resistant potato varieties, addition of organic amendments to increase the activity of antagonistic microorganisms, and by use of biological-control agents (Sethi and Gaur, 1986). However, the endoparasitic nature of the nematode, deposition of eggs into a gelatinous eggs mass (in RKN) and the female turning into a hard cyst encompassing eggs within (in PCN) makes them difficult organisms to manage. Occurrence of physiologic variations (Krishna Prasad, 1996; Mathur and Krishna Prasad, 1998; Wajid Khan and Khan, 1998) complicates the use of resistant varieties. Experience shows that these nematodes need to be managed by adopting several plant protection strategies. Cultural practices Cultural management basically involves all crop- husbandry practices that deprive the nematodes from infesting potato thereby reducing their population (Evans and Stone, 1977; Evans, 1993). The most common practice is use of healthy seed, crop rotation and intercropping with antagonistic plants. Surveys conducted to document species- complex, distribution and subsequent mapping of nematode intensity, have helped identify nematode-free zones for seed- potato production in India (Fig 2 and 3). This has helped in obtaining healthy seed potatoes and thus potato from North- West plains of Punjab and Haryana States and the high hilly districts of Kullu, Kinnaur and Lahual & Spiti in Himachal Pradesh are more favored as a source for reducing RKN infection. On the other hand, potato from Nilgiris and Kodaikanal hills is avoided, to restrict the movement of PCN. Growing non-host crops such as maize, wheat and the trap-crop, marigold helps minimize RKN in northern India (Gill, 1974; Deshraj, 1983). A two-year crop rotation and application of nematicide (carbofuran @ 2 kg ai /ha in two equal splits, once at planting and the other at earthing- up) increased potato yields by 45% and brought down RKN tuber-infection by 96% in Shimla hills (Krishna Prasad, 1986). Crop rotation with non-solanaceous vegetables (beetroot, cabbage, carrot, cauliflower, french bean, garlic, Krishna Prasad J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 99 radish, turnip, etc.) between September-November reduced PCN cysts and its propagules in South Indian hills, ranging from 24 and 76% respectively (Table 4). A three and four year rotational sequence with a nematode susceptible and resistant potato brought down PCN by 98 to 99% and increased yields by 65% in Nilgiri hills (Krishna Prasad, 2000). These crop rotations and inter-cropping of potato with French bean or wheat reduced nematode infestation by 35%, while, nutritive value of the soil increased (Manorama et al, 2003). Exposing soil to summer heat, fallowing, mulching, trap-cropping, addition of organic amendments to increase the activity of antagonistic organisms in the soil are other methods of nematode management in potato. Breeding for resistance Use of host-resistance is the most sustainable nematode management strategy and has been exploited in potato nematode management (Stone and Turner, 1983). Research on breeding for nematode resistance in India started for RKN in 1961 and for PCN in 1968 at the Central Potato Research Institute, Shimla and its sub-station at Ootacamund. High degree of resistance in several tuber- bearing Solanum species is available (Table 5) both for RKN and PCN (Birhman et al, 1998; Gaur et al, 1999). Continuous breeding and selection of resistant lines of potato brought to light existence of variations within the species of RKN and PCN which were designated as biotypes and pathotypes. Pathotype-specific, major genes control resistance to PCN (Kort et al, 1977) and non-specificTable 4. Effect of growing potato and other crops on cysts and propogules of PCN Initial population : Cysts 250/100 ml soil Eggs and larvae : 85 /cyst Crop Build-up Build up Calculated Per cent index * index* of CPR** reduction of cysts eggs and value over larvae susceptible potato var. at harvest Susceptible 4.982 2.551 12.71* potato var. Resistant 0.582 0.454 0.26 97.9 potato var. French bean 0.612 0.752 0.46 96.4 Cabbage 0.514 0.385 0.22 98.4 Carrot 0.537 0.392 0.19 98.3 Garlic 0.628 0.422 0.26 97.9 Maize 0.734 0.704 0.51 95.9 Wheat 0.716 0.795 0.57 95.5 Oat 0.695 0.681 0.47 96.3 Fallow 0.915 0.860 0.76 93.9 *Build-up index calculated as the ratio of final population over initial population ** CPR Calculated as Cumulative Percent Reduction over initial population Table 5. Nematode resistance available in tuber-bearing Solanum species (compiled from CPRI Annual Reports) Species Number of accessions Number of accessions resistant to RKN resistant to PCN Solanum acaule 3 2 S. acroscopicum 1 — S. agrimonifolium 1 — S. ajanhurri 1 — S. bolviense 2 — S. brevicaule 1 — S. bulbocastanum 1 1 S. cardiophyllum 3 — S. chacoense 14 2 S. chrenbergii — 1 S. chaucha 1 — S. curtilobum 3 — S. demissum 7 2 S. famatinae 1 1 S. fendleri — 1 S. hougasii 1 — S. infundibuliforme 1 — S. jamesil 1 — S. gourlayi — 1 S. grandarillasii 1 — S. kurtzianum 2 3 S. leptophyes 2 — S. lignicaule 1 — S. maglla 2 — S. microdontum 1 3 S. multidissectum 1 2 S. ochranthum 1 — S. oplocense — 2 S. phureja 2 2 S. pinnatisectum 1 — S. raphanifolium 4 — S. recho 1 — S. sanitaerosae 1 — S. sparsipillum 2 3 S. spegazzinii 5 9 S. stenophyllidium 1 — S. stenotonum 1 — S. stoloniferum 8 — S. sucrense — 2 S. tarifense — 1 S. tuberosum spp andigena 28 168 S. tuberosum 16 13 S. vallis mexici 1 — S. vernei 2 5 Inter varietal Hybrids 161 287 Imported accessions 75 22 Management of potato nematodes J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 100 polygenes to RKN (Harikishore et al, 1977). An intervarietal hybrid HC-294 possessed resistance to M. incognita and several commercial potato varieties showed reduced development and reproduction (CPRI, 1986). High degree of PCN resistance available in S. vernei (clone 62-33-3) has been used for developing a PCN resistant variety ‘Kufri Swarna’ (Fig 12) at the Nilgiris (CPRI, 1986). This variety offers better management of PCN as it is resistant to pathotypes Ro 1 and Pa 2 (Maximum occurrence). Continuous cultivation of resistant varieties helps build up other pathotypes (Krishna Prasad, 1996). Therefore, it has to be alternated often with a susceptible potato variety. South Indian hills, which grow potato throughout the year, are also endemic to the late blight (LB) pathogen, Phytopthora infestans (Krishna Prasad and Latha, 1999), and hence, potato varieties should have combined resistance to PCN and LB (Fig13). Potato breeding and evaluation resulted in developing 21 promising advance hybrids (Fig 14) with combined resistance to PCN and LB (Krishna Prasad et al, 2001; Joseph et al, 2003). Two advance hybrids OS/93- Fig 12. PCN resistant variety Kufri Swarna Fig 13. Growth comparison in potato varieties in healthy & nematode-sick plots Fig 14. View of promising advance hybrids with combined resistance to PCN and LB at CPRS, Ooty D204 and OS/94-L956, have entered Adoptive Research Trials in farmers’ fields (Fig 15; Joseph and KrishnaPrasad, 2005). One of the selections, E/79-42, has been registered with NBPGR as an excellent male source, with combined resistance to PCN and LB (Krishna Prasad, 2006). Chemicals in nematode management Initially, DD (1-3 Dichloropropane1-2 dichloropropene), EDB (Ethylene di bromide), MBr (Methyl bromide), DBCP (Dibromochloropropene), Dorlone (a mixture of DD and EDB) were used for controlling RKN and PCN. Massive chemical-control attempt was also made under the Indo-German Nilgiris Development Project during 1971-75 to check PCN. The treatment was made mandatory under the Tamil Nadu Pest Act 1971 and all the infested fields at that time in Nilgiris were treated with 30 kg ai/ha of Fensulfothion (Dasanit 10 % G) in the first year, followed by 15 kg ai/ha in the subsequent years. Nearly 1000 tons of 10% Fensulfothion was used treating about 3100 hectares of annual cropped area, over a period of five years (Seshadri, 1978). Thus, each hectare, on average, received 325 kgs of the pesticide. In spite of LB resistant PCN susceptible LB susceptible PCN susceptible LB susceptible PCN resistant Fig 15. Adoptive research trials in farmers’ fields in the Nilgiris Krishna Prasad J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 101 this massive application of pesticides, PCN continued to be one of the limiting factors for potato production in the Nilgiris (Table 6; CPRI, 1985). At present, application of Carbofuran at 2 kg ai/ha is recommended to keep nematodes at below the economic threshold levels of less than 10 eggs or larvae per 100 ml of soil at the time of planting potato (Krishna Prasad, 2000). This will help reduce the initial nematode inoculum in soil and also reduce build-up of pathotypes that multipliy on the resistant cv. ‘Kufri Swarna’(Krishna Prasad, 2007b). Biocontrol agents for nematode management Nematode management in potato by bioagents has not been exploited to its full potential (Crump, 1989; Crump and Flynn, 1992). The fungus, Paecilomyces lilacinus, is effective against root knot nematodes on potato (Jatala, 1985) and was extensively used for RKN management in other crops under the International Meloidogyne Project (Sasser, 1989), while, the endomychorhhizal fungi, Glomus fasciculatus and G. mossae, offer possibilities for nematode management, especially RKN, in India (Krishna Prasad, 1993). Two bioagents, Paecilomyces lilacinus and Pochonia chlamydosporia, in talc formulations, were tested for PCN management under field conditions, with and without the nematicide (Krishna Prasad and Nagesh, 2007). Both these organisms (Fig 16) were able to reduce cysts and propagules Fig 16. Bio-agent growing on potato cyst nematodes Fig 17. Potato cyst mematode-sick plots at CPRS, Ooty Table 6. Quantity of Dasanit 10%G used in the Nilgiris for PCN control Year Hectares Dasanit 10 % G treated used in kg 1970 1220 3,28,518 1971 1400 2,23,639 1972 1564 1,99,425 1973 1227 1,11,246 1974 1874 1,37,150 Total 9,99,978 kg (1000 tonnes) by 46 to 49% while increasing marketable-potato yields by 40 to 70%. Similarly, Pseudomonas flourencens, in combination with Paecilomyces lilacinus, brought about substantial yield increase upto 70% while reducing PCN (Malavika et al, 2008). The rapidity with which many of these nematode biocontrol agents have been used in the Indian agriculture for management of several parasitic nematodes like RKN, in vegetables (Rao et al, 1997, 1998; Rao, 2004), ornamentals (Rao et al, 2003) and reniform and burrowing nematodes in cotton (Shivakumar et al, 2004) and banana (Karuna et al, 2004) has shown that indigenously isolated and multiplied biocontrol agents will become an important component of nematode management in potato as well (Walia, 2004, Krishna Prasad, 2007a). Integrated management Experience has shown that neither RKN nor PCN can be eradicated once these establish in a locality (Sethi and Gaur, 1986; Hamid and Alam, 1998). Their similarities in population dynamics, biology and life cycle facilitate integrated management of RKN and PCN in potato. Hence, these nematodes need to be managed by combining several plant-protection strategies (Kamra and Dhawan, 1998). RKN in the north Indian hills is being managed by application of Carbofuran 3G at 1 or 2 kg ai/ha at potato planting (Deshraj, 1983); two-year crop rotation with wheat or maize and seed certification after inspection for root knots on seed tubers (Krishna Prasad, 1986, 1993). This has helped keep juveniles of RKN at less than the threshold level of 10 to 20 larvae /100 g of soil sample. Integrated PCN management practice was standardized in a nematode sick plot (Fig. 17) over a period of eight years involving crop rotation, intercropping, host resistance and nematicide treatment at potato planting Management of potato nematodes J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 102 (Krishna Prasad, 2001). The treatments incorporated were potato cultivar Kufri Swarna resistant to PCN, potato cultivar Kufri Jyothi-PCN susceptible, locally grown vegetable cultivars of cabbage, carrot, French bean, garlic, pea, radish, and Carbofuran at 2 kg ai /ha. A PCN sick plot with initial population of 42 -50 cysts /100 soil harboring about 1500-2000 eggs and larvae was sub-divided into 72 sub-plots and the above crops were grown during April- August. The potato crop received nematicide treatment. Nematode build-up was monitored during the crop period and crops were alternated in the subsequent year. Each year, after the harvest of the main crop in sub-plots, wheat and fodder oat were planted. (Fig 18). This further reduced nematode populations by 20 to 24%, in addition to utilizing residual moisture in the soil. Nematode resistant potato and other vegetables were able to bring down the nematode population by 82% in the very first year, with an accumulated reduction of upto 99.5% following pesticide treatment and crop rotation. At the same time, increase in nematode population in the susceptible potato variety was 3.2 times for cysts and 6.3 times for eggs and larvae, resulting in 65% yield-reduction. Cabbage and carrot grown after potato harvest reduced initial nematode populations and increased the potato yield the subsequent year. Studies indicated that even a three- year crop rotation of susceptible potato is sufficient to get economical yields when it is rotated with resistant potato or cabbage or carrot. However, there is a need to resort to minimum application of pesticide (2 kg ai/ha of Carbofuran) before the nematode-resistant potato can be grown, to keep the initial nematode population low and to avoid build-up of the less prevalent pathotypes. This cropping sequence has given 28 to 30 t/ha in PCN susceptible potatoes and 31 to 34 t /h in the resistant potatoes. Now, farmers in the Nilgiris are following this crop management schedule (Fig 19). Other nematodes Several parasitic nematodes such as the lesion, spiral, stunt and reniform nematodes are constantly encountered in surveys on potato fields (Gill, 1974). Not much information was available hitherto on their role as pathogens in potato (Krishna Prasad, 1984), hence, their relative occurrence was quantified in potato based croping system at Nilgiris(Table 7). Pathogenicity of Quinslcius capitatus, a stunt nematode frequently occurring in the Himalayan mid-hills (Krishna Prasad and Sharma, 1985), and the spiral nematode (Helicotylenchus dihystera) prevalent in all the potato- growing tracts of India, was established on potato (Krishna Prasad and Rajendran,1990).The stunt nematode reduced 14 to 29% tuber yield, while, the spiral nematode accounted for 9 to 27% tuber yield reduction. Both these nematodes, polyphagous ectoparasites commonly encountered in all the potato-growing localities, are potential pests of potato, if Fig 19. A view of farmers practising IPM in the Nilgiris Fig 18. IPM practices for nematode management in potato Table 7. Build-up of ectoparasitic nematodes on different crops in the Nilgiris Crop Duration Spiral Lesion Total in days nematode nematode EPN Potato 110 7.82 5.35 6.58 Bean 75 3.45 2.85 3.15 Cabbage 115 6.45 4.85 5.65 Carrot 125 4.85 3.50 4.18 Garlic 105 4.50 3.30 3.90 Maize 135 7.60 5.20 6.40 Wheat 110 6.00 4.85 5.42 Oot 120 5.05 4.25 4.65 Mean Build-up 5.71 4.27 4.99 Index* * *Build-up index calculated as the ratio of final population over initial population Krishna Prasad J. Hortl. Sci. Vol. 3 (2): 89-106, 2008 103 RKN or PCN are to be managed by host resistance alone. Studies on build-up of spiral and lesion nematodes in potato fields at Ootacamund showed that ectoparasitic nematodes preferred potato, followed by maize, cabbage and wheat in that order. Other parasitic nematodes of importance in potato are the pin (Paratylenchus spp.), reniform (Rotylenchulus spp.) and lesion (Pratylenchus spp.) nematodes. However, management practices followed for PCN or RKN reduce these pests (Krishna Prasad, 2007b). Conclusion Potato, an important crop grown in India, throughout the country, is associated with about 90 species of plant parasitic nematodes belonging to 38 genera. Among these, root-knot nematodes and cyst nematodes are important constraints in crop productivity. Root-knot nematodes are present throughout the country, while, cyst nematodes are restricted to the hilly regions of Tamil Nadu. The endoparasitic nature of life cycle in these nematodes poses difficulties in their management. Further, physiologic variation in these nematodes makes host resistance often a failure. Therefore, an integrated nematode management (INM) is adopted in both the cases. The root-knot nematode in North India is managed using nematode-free seed tubers, a two-year crop rotation with maize or wheat, and, with nematicide (carbofuran @ 2 kg ai/ha) application in split doses. The cyst nematode in Tamil Nadu hills is managed by following a three-year crop rotation with vegetables, intercropping with beans or wheat, using nematode resistant potatoes and by application of the nematicide in split doses. Adoption of INM for these nematodes has proved efficient and economical in Himachal Pradesh and Tamil Nadu hills. Few indigenously multiplied biocontrol agents like Paecilomyces lilacinus, Pochonia chlamydosporia and Pseudomonas flourencens offer excellent opportunity for incorporation into this nematode-management system. ACKNOWLEDGEMENT The author is grateful to Dr. N.M. Nayar, Dr. G.S. Shekhawat, and Dr. S.M. Paul Khurana, ex-Directors and Dr. S.K. Pandey, present Director, Central Potato Research Institute, Shimla, for their encouragement and providing facilities to work on nematode problems in potato. The author wishes to thank the association of Drs. M. Latha, C.P. Gajaraja, T.A. Joseph, K. Manorama, D.B. Singh and G. 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